Here we can show some examples of working configuration for Asterisk's SIP channel driver when Asterisk is behind NAT (Network Address Translation).
If you are migrating from chan_sip to chan_pjsip, then also read the NAT section in Migrating from chan_sip to res_pjsip for helpful tips.
Asterisk and Phones Connecting Through NAT to an ITSP
We are assuming you have already read the Configuring res_pjsip page and have a basic understanding of Asterisk. For this NAT example, the important config options to note are local_net, external_media_address and external_signaling_address in the transport type section and direct_media in the endpoint section. The rest of the options may depend on your particular configuration, phone model, network settings, ITSP, etc. The key is to make sure you have those three options set appropriately.
This is much like the external_media_address setting, but for SIP signaling instead of RTP media. The two external* options mentioned here should be set to the same address unless you separate your signaling and media to different addresses or servers.
Determines whether media may flow directly between endpoints
Together these options make sure the far end knows where to send back SIP and RTP packets, and direct_media ensures Asterisk stays in the media path. This is important, because our Asterisk system has a private IP address that the ITSP cannot route to. We want to make sure the SIP and RTP traffic comes back to the WAN/Public internet address of our router. The sections prefixed with "sipus" are all configuration needed for inbound and outbound connectivity of the SIP trunk, and the sections named 6001 are all for the VOIP phone.
[transport-udp-nat] type=transport protocol=udp bind=0.0.0.0 local_net=192.0.2.0/24 local_net=127.0.0.1/32 external_media_address=198.51.100.5 external_signaling_address=198.51.100.5 [sipus_reg] type=registration transport=transport-udp-nat outbound_auth=sipus_auth server_uri=sip:gw1.example.com client_uri=sip:[email protected] contact_user=19998887777 retry_interval=60 [sipus_auth] type=auth auth_type=userpass password=************ username=1112223333 realm=gw1.example.com [sipus_endpoint] type=endpoint transport=transport-udp-nat sectioncontext=from-external disallow=all allow=ulaw outbound_auth=sipus_auth aors=sipus_aor direct_media=no from_domain=gw1.example.com [sipus_aor] type=aor contact=sip:gw1.example.com contact=sip:gw2.example.com [sipus_identify] type=identify endpoint=sipus_endpoint match=203.0.113.1 match=203.0.113.2  type=endpoint sectioncontext=from-internal disallow=all allow=ulaw transport=transport-udp-nat auth=6001 aors=6001 direct_media=no  type=auth auth_type=userpass password=********* username=6001  type=aor max_contacts=2
For Remote Phones Behind NAT
In the above example we assumed the phone was on the same local network as Asterisk. Now, perhaps Asterisk is exposed on a public address, and instead your phones are remote and behind NAT, or maybe you have a double NAT scenario?
In these cases you will want to consider the below settings for the remote endpoints.
IP address used in SDP for media handling
At the time of SDP creation, the IP address defined here will be used as
the media address for individual streams in the SDP.
NOTE: Be aware that the 'external_media_address' option, set in Transport
configuration, can also affect the final media address used in the SDP.
Enforce that RTP must be symmetric. Send RTP back to the same address/port we received it from.
Force RFC3581 compliant behavior even when no rport parameter exists. Basically always send SIP responses back to the same port we received SIP requests from.
Determines whether media may flow directly between endpoints.
Determine whether SIP requests will be sent to the source IP address and port, instead of the address provided by the endpoint.
Clients Supporting ICE,STUN,TURN
This is really relevant to media, so look to the section here for basic information on enabling this support and we'll add relevant examples later.